A typical EDM apparatus 1 of the type described above is illustrated in FIG. 1 and includes an electrode 10 supported by a shaft 12 and suspended over a tank 14. Shaft 12 is connected at its opposite end to a cylinder 16, which is positioned by hydraulic fluid provided by a motor 20 via an electrically controlled servo valve 18. Electrode 10 opposes a workpiece 22 across a machining gap G. Tank 14 is filled with a machining solution 24, whose level in the tank insures that machining gap G is always filled with machining solution 24.
Electrode 10 and workpiece 22 are serially connected by a pair of leads to output terminals of machining power source 30, which includes a DC power supply 32 with a rated output of E volts (V), a switch 34 for switching the power source 30 ON and OFF, an oscillator 36 for controlling the operation of switch 34 and a current limiting resistor 38 with a resistance value of R. Power source 30 supplies an interelectrode voltage V.sub.G to the pair of leads so that a switching current I is applied between electrode 10 and workpiece 22.
The switching current I is represented by the expression I=(E-V.sub.G)/R, where V.sub.G is in the range of about 20 to 30 V during an arc discharge period, 0 V during a short circuiting period and E V during periods when no arc discharge occurs. If the interelectrode voltage V.sub.G is detected and averaged by a smoothing circuit 40, the machining gap G can be controlled in response to the averaged value of the interelectrode voltage V.sub.G. More specifically, when the machining gap G is wide, a discharge across the machining gap does not occur and the average voltage, hereinafter denoted V.sub.ave, becomes high, i.e., approaches E. When the gap is narrow, a short circuit between the electrode 10 and workpiece 22 can occur, which results in a reduction in the average voltage V.sub.ave. Accordingly, when the value of V.sub.ave is compared with a reference voltage V.sub.REF, the magnitude and polarity of the difference between these two voltages can be applied to servo 18 via an amplifier 42 to position electrode 10 with respect to workpiece 22. Thus, the difference between V.sub.ave and V.sub.REF, e.g., control voltage V.sub.C, can be used to control the machining gap G at a substantially constant value.
However, those of ordinary skill in the art will appreciate that controlling the machining gap alone does not amount to controlling the operating parameters of the EDM apparatus 1 to optimize the material removal rate during a machining operation. The change in a wide variety of physical conditions in EDM apparatus 1 can effect the interelectrode voltage V.sub.G. Sludge accumulation in machining gap G, for example, can result from a low circulation or flow rate of machining solution in machining gap G and can decrease the interelectrode impedance, thus causing a decrease in V.sub.G. Also, carbonization of electrode 10 due to thermal decomposition of the machining solution 24 or other causes can increase the possibility of arcing or abnormal machining, and lower the value of V.sub.G. Therefore, even when an optimal material removal rate for an electrode/workpiece combination has been determined by experimentation, EDM apparatus 1 cannot be controlled to achieve that optimal removal rate.